ON ELECTROLYSIS. 727 
position and of polarisation ; unless, of course, rigorous proof can be given 
that its conduction is metallic, which has in no case happened yet. 
Electrolytes seem necessarily fluid, and it is difficult to imagine the 
locomotion of atoms which accompanies electrolysis to go on in a solid 
body. Mr. Shelford Bidwell, Dr. Silvanus Thompson and others con- 
sider they have found solid electrolytes, but it behoves us to be very 
careful in accepting such an idea; extreme viscosity there may be, as in 
hot glass, but not the true rigidity of a solid, unless certain proof is 
forthcoming. 
The tests of electrolytic conduction are these four— 
1. Visible decomposition. 
2. Polarisation. 
3. Non-agreement with Volta’s series law. 
4, Transparency. 
We may thus pretty safely distinguish electrolytes from metals, but to. 
certainly distinguish electrolytes from dielectrics is not always easy. 
True a dielectric does not conduct, but it may break down. How to 
distinguish an electrolyte from a weak dielectric, as, for instance, rare 
air: thisis not easy. An initial E.M.F. is needed to commence dis- 
ruptive discharge, but that may be easily confused with electrolytic 
polarisation of electrodes: indeed some facts suggest that there is a 
considerable surface or boundary resistance which opposes the passage 
of electricity from metal into rare air and may altogether stop it when 
yery small terminals are used. 
Second meaning of I. 
Now pass to the second aspect of the question, what is an electro- 
lyte, viz. (b) Is a particular substance the electrolyte when mixed with 
other matter ? 
Ifa voltameter had a glass partition with a hole in it, no supporter of 
the view that glass was an electrolyte would contend that the glass con- 
ducted some of the current. Similarly with a water partition, or with 
any water existing in a mass in any part of the vessel. Non-conductivity 
or bad conductivity has thus everything to say to this question, What 
is the electrolyte ? 
But then, in a solution of acid or salt water, it is not merely mixed 
with the substance, it is combined with it; and it is very possible that 
water in this state may conduct readily enough. Information on this head, 
sure and definite as it seems to me, is given by the simple fact that pure: 
H,O and pure liquid HCl both almost insulate when separate, but 
conduct well enough when mixed. 
There are, then, four hypotheses concerning the apportionment of the- 
current among the substances in a salt solution. 
(1) That the salt alone conducts. 
(2) That the water alone conducts. 
(38) That the salt and the water share the conduction between them. 
(4) That neither salt nor water exist, but that a hydrate is formed, 
and that this conducts and is decomposed as a whole. 
To decide whether when substances are mixed both conduct the cur- 
rent, Hittorf mixed KCland KI in various proportions, and concluded. 
that the eurrent always travelled through both salts. Buff has made 
many similar experiments, and agrees. Gore has deposited brass from a 
